With the recent popularization of computers, inkjet printers are much used for printing on paper, films, cloths and others, not only in office work but also for home use.
The inkjet recoding method includes a system of applying pressure to ink drops by the use of a piezoelectric device to thereby make the ink drops jet out, a system of thermally bubbling ink to jet the ink drops, a system of using ultrasonic waves, and a system of sucking and jetting ink drops by electrostatic force. For the ink compositions for such inkjet recording, usable are aqueous inks, oily inks and solid (hot-melt) inks. Of those, aqueous inks are most used because of their productivity, handlability, odorlessness and safety.
Colorant to be used in such inkjet recording inks must satisfy the following requirements: Its solubility in solvent is good; it enables high-density recording; its hue is good; its fastness to light, heat, air, water and chemicals is good; it well fixes in image-receiving material and hardly bleeds out; its storage stability in inks is good; it is not toxic; its purity is high; and it is inexpensive and is readily available. However, it is extremely difficult to seek such colorant that satisfies all these requirements on a high level.
Various dyes and pigments have been already proposed for inkjet, and are now in practical use. At present, however, no one knows colorant that satisfies all the requirements. Dyes and pigments heretofore well known in the art such as those listed in Color Index (C.I.) could hardly satisfy both the color hue and the fastness, which inks for inkjet need.
As dyes of improved fastness, azo dyes derived from aromatic amines and 5-membered heterocyclic amines are proposed, as in JP-A 55-161856. However, these dyes have an unfavorable hue in a region of yellow and cyan, and are therefore problematic in that they worsen color reproducibility.
JP-A 61-36362 and 2-212566 disclose an inkjet recording ink that may satisfy the two requirements of color hue and light fastness. However, the dyes used in these patent publications are unsatisfactory in point of their solubility in water when they are to be in water-soluble inks. In addition, when the dyes described in these patent publications are used in water-soluble inks for inkjet, they are problematic in point of the wet heat fastness thereof.
To solve these problems, compounds and ink compositions are proposed as in JP-T 11-504958 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application). In addition, an inkjet recording ink that comprises a pyrazolylanilinazo dye for improving the color hue and the light fastness of the ink is described in JP-A 2003-231850. However, these inkjet recording inks are all unsatisfactory in point of the color reproducibility thereof and the fastness of the outputted images.
Further, it has been found that, when images are printed on photograph-level, inkjet-dedicated glossy paper and when the prints are put up in the room, then the storability of the images thereon is often extremely bad. We, the present inventors presume that this phenomenon may be caused by any oxidizing gas in air such as ozone. If the prints are kept in glass frames or the like so as to protect them from exposure to air streams, then the phenomenon may hardly occur, but it restricts the use condition of the prints.
The phenomenon is extremely remarkable in photograph-level inkjet-dedicated glossy paper, and this is one serious problem with the current inkjet recording system in which the photograph-level quality of the images to be formed is one important characteristic of the system.
Heretofore, we, the present inventors have investigated various dyes of good color hue and good fastness, and have developed those favorable for colorants for inkjet recording. However, we have known that the single use of water-soluble dye alone could not attain satisfactory fastness.
In addition, we have further investigated dye-containing, aqueous inkjet inks, and have found a problem with them in that, in a weather test of gray continuous-tone images and portraits, low-density yellow parts are readily faded to break the color balance of the images.
On the other hand, when the solvent remaining in ink is removed insufficiently, then it causes a problem in that the inkjet print density lowers during storage (for example, yellow parts may look faded or black parts may change). In particular, it is important to prevent the density reduction at high temperature and high humidity.